May 1, 2013 / Vol. 38, No. 9 / OPTICS LETTERS

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Brillouin scattering signal in polymer optical fiber
enhanced by exploiting pulsed pumpwith multimode-fiber-assisted coupling technique
Yosuke Mizuno,* Neisei Hayashi, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan*Corresponding author: ymizuno@sonic.pi.titech.ac.jp
Received February 6, 2013; revised March 11, 2013; accepted March 25, 2013;
posted March 27, 2013 (Doc. ID 185028); published April 26, 2013
A cost-effective technique for coupling a polymer optical fiber (POF) with 50 μm core diameter to a silica singlemode fiber (SMF) with 8 μm core diameter is proposed, which can, by exploiting a multimode fiber with 50 μm core
diameter, avoid the damage or burning at the butt-coupled POF/SMF interface. Using this coupling technique, we
also show that the Brillouin signal in a POF can be enhanced by combined use of pulsed pump and an erbium-doped
fiber amplifier. When the pulsed pump with average optical power of 18 dBm (63 mW), duty ratio of 15%, and pulse
period of 2 μs is launched into a 200 m-long POF, 4 dB enhancement of the Stokes power is obtained compared tothat with 18 dBm continuous wave pump. The relatively small enhancement is probably caused by the high Brillouin threshold of POFs. The Stokes power dependence on duty ratio is nonmonotonic, which might originate froma longer phonon lifetime in POFs than that in silica SMFs. © 2013 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (130.5460) Polymer waveguides; (290.5830) Scattering, Brillouin.
http://dx.doi.org/10.1364/OL.38.001467

Brillouin scattering in optical fibers [1] has been the subject of intense research for several decades as a prospective mechanism for various devices and systems [2–9].
Since glass optical fibers are fragile and need to betreated with care, we have focused on Brillouin scattering in polymer optical fibers (POFs) with such high flexibility that they can withstand >50% strain. Up to now,
especially for sensing applications, we have investigatedsome fundamental Brillouin properties of perfluorinatedgraded-index POFs at 1.55 μm, including Brillouin gain
coefficient [10], Brillouin threshold power [10], as well as
Brillouin frequency shift [10] and its dependences on
strain (relatively small strain [11] and large strain [12])and temperature [11]. These results indicate that Brillouinscattering in POFs is potentially applicable both to highprecision temperature sensing [11] and to large-strain
sensing [12]. However, the Stokes power in POFs is quite
low because of their large core diameters (>50 μm),
which needs to be enhanced for more detailed investigation of their Brillouin gain spectra (BGSs) [13–15].
Several methods have been developed to enhance theBrillouin signal in POFs. The Stokes power was enhanced simply by employing long POFs [16], but the useof POFs longer than ∼50 m has been proved to be of
scarcely any effect due to their high propagation loss
at 1.55 μm resulting in short effective length. Anotherway is to induce stimulated Brillouin scattering with apump-probe technique [17], in which detailed evaluationof the BGS was, however, difficult because it was easily
influenced by lock-in configurations. Exploiting the nonlinearity of Brillouin scattering, we have also demonstrated that the Stokes signal in silica single-mode fibers(SMFs) can be drastically enhanced by use of pulsed
pump and a low-power erbium-doped fiber amplifier(EDFA) [18]. This method was, however, inapplicable tothe enhancement of the Stokes signal in POFs, because
the butt-coupled POF/SMF interface was easily damaged
or burnt due to the extremely high peak power of thepulsed pump. Inserting a multimode fiber (MMF) with
0146-9592/13/091467-03$15.00/0

an appropriate mode-field diameter at the POF/SMF interface [19] might be used to avoid the damage, but, whena POF with 50 μm core diameter is used, the corresponding optimal core diameter of the MMF becomes a valuethat is not commercially standardized.
In this Letter, we develop a new POF-to-SMF couplingtechnique, which can, with the assistance of an MMF,
minimize the damage at the POF/SMF interface. Then weenhance the Brillouin signal in a POF using pulsed pump
and an EDFA, which method was unfeasible without the
MMF-assisted coupling. The obtained enhancement in the
Stokes power is 4 dB, which is relatively small because of
the high Brillouin threshold of the POF. The Stokes power
dependence on duty ratio is found to be nonmonotonic,
which might originate from a longer acoustic phonon
lifetime in POFs than that in silica fibers.
In general, the dependence of Stokes power on continuous wave (CW) pump power is nonlinear [1]; namely,
the Stokes power begins to grow drastically via electrostriction when the pump power is higher than Brillouinthreshold power P th . When pulsed pump with its peak
power higher than the threshold is employed, temporally
averaged Stokes power can be enhanced compared tothat with CW pump with the same average power [18].
Such pulsed pump with high peak power can be generated with a low-power EDFA, which typically has smallsignal gain of around 20–40 dB and saturation output
power of around 10–20 dBm. So long as the average output power is lower than the saturation output power, an
incident optical pulse even with several-milliwatt peak
power experiences the small-signal gain. This condition
can be satisfied by sufficiently decreasing the pulse duty
ratio, leading to the generation of pulsed pump with
(sub-)Watt-range peak power.
In our previous report [18], by employing the pulsed
pump with average optical power of 13 dBm, duty ratioof 20%, and pulse period of 2 μs, the Stokes power in a
1 km-long silica SMF (P th  17 dBm  50 mW) was drastically enhanced by 25 dB. Each parameter was defined© 2013 Optical Society of America

1468

OPTICS LETTERS / Vol. 38, No. 9 / May 1, 2013as shown in Fig. 1. In contrast, when the pulsed pumpwith average power of 19 dBm, duty ratio of 10%, andpulse period of 2 μs was launched into a 2 m-long silica
SMF (P th  44 dBm  25.1 W), the Stokes power was
also raised probably due to the nonlinearity of the Stokes
power dependence on CW pump power even within therange below P th , but only by 0.25 dB. Thus, the effectiveness of this method is highly dependent on the Brillouinthreshold of the fibers under test (FUTs), which should
be lower than the peak power of the pulsed pump.
A 200 m-long perfluorinated graded-index POF withnumerical aperture of 0.185, core refractive index of∼1.35, and propagation loss of 250 dB∕km at 1.55 μm
was used as an FUT (Note that almost the same measurement results are expected irrespective of the POF length,
providing it is longer than ∼50 m). Following the methodthat takes the multimode correction into consideration[16,20], its Brillouin threshold was estimated to be
40.2 dBm (10.4 W). The experimental setup shown in
Fig. 2 was similar to that previously used in [18], which
was based on self-heterodyne detection for highresolution BGS monitoring. All the optical paths except
for the FUT were composed of silica SMFs. A distributedfeedback laser diode at 1.55 μm was used as a lightsource. Pulse conversion of the pump was performed
with a LiNbO3 intensity modulator having an extinction
ratio of over 20 dB. A low-power EDFA with small-signal
gain of 35 dB was used to amplify the pulsed pump. The
relative polarization state of the Stokes and the reference
beams was optimized with polarization controllers.
Conventionally, POFs were butt-coupled to silica SMFs
for Brillouin measurement [10], which was subject tophysical damage or burning, resulting in inapplicabilityof high-peak-power optical pulse injection. To resolve
this problem, we developed a new coupling technique exploiting an MMF, the concept of which is schematicallyshown in Fig. 3. The conventional method suffers fromthe differences in material and in core diameter simultaneously at the POF/SMF interface. Damage is caused not
only by the difference in core diameter but also by theslight difference in surface roughness, which cannot
be removed by splicing in this case because the two materials to be spliced are quite different (polymer andFig. 1.

Definitions of parameters related to optical pulse train.

Fig. 2. Experimental setup. DC, direct current; EDFA, erbiumdoped fiber amplifier; ESA, electrical spectrum analyzer; IM,intensity modulator; ISO, isolator; LD, laser diode; PC, polarization controller; PD, photo detector; VOA, variable opticalattenuator.

Fig. 3.

Concept of MMF-assisted coupling.

glass). In contrast, in the new coupling method, a silica
MMF with the same core diameter as that of the POF isinserted between the POF and the silica SMF, and spliced
to the SMF by arc fusion (both silica glass). Then, thedamage originating from the difference in surface roughness can be avoided at the MMF/SMF interface, while nodifference in core diameter can mitigate the damage at
the POF/MMF interface. To verify whether this coupling
method itself has an effect of enhancing the Brillouinsignal, BGSs in the POF were measured with and without
the MMF-assisted coupling using 12 dBm CW pump, asshown in Fig. 4. The Brillouin frequency shift wasapproximately 2.8 GHz, which agrees with our previous
report [10]. The Stokes power with the MMF was only
slightly (∼0.25 dB) higher than that without the MMF,
which indicates that the MMF-assisted coupling itself
has hardly any influence on the Stokes power in POFs.
Further research is required to clarify the physical process of reflection and transmission of electromagnetic andacoustic waves along the POF/MMF/SMF structure.
Using the MMF-assisted coupling to avoid the damage
at the POF/SMF interface, we applied to the 200 m-long
POF the method for enhancing the Stokes signal using
pulsed pump. The average power of the pump was set
to 18 dBm. The pulse period was fixed at 2 μs, and the
duty ratio was varied from 100% (CW) to 5%. During
the experiment, the POF/MMF and MMF/SMF interfaces
remained undamaged. The microscopic images of the
POF ends with and without the MMF after all the measurements are shown in Figs. 5(a) and 5(b), respectively.
Burning was observed at the core in Fig. 5(b), which was
successfully avoided in Fig. 5(a).
The measured BGS dependence on duty ratio in the
POF is shown in Fig. 6(a). With decreasing duty ratio,
the Stokes power was raised, probably because of thenonlinear dependence of the Stokes power on CW pump
power in the range of <P th . Figure 6(b) shows the Stokes
power dependence on duty ratio. The error bars were
calculated based on the signal fluctuations in ten minutes. As the duty ratio was reduced, the Stokes power

Fig. 4. Measured BGSs with and without MMF-assisted
coupling.

May 1, 2013 / Vol. 38, No. 9 / OPTICS LETTERS

Fig. 5. Microscopic images of POF ends after measurements(a) with and (b) without MMF-assisted coupling.

was enhanced, which is the same behavior as that with along silica SMF [18]. At 15% duty ratio, a maximum enhancement of 4 dB was obtained. This value is muchsmaller than that with a silica SMF [18], which is natural
because the calculated peak power of ∼26.3 dBm(420 mW) is much lower than the Brillouin thresholdof 40.2 dBm (10.4 W). More drastic enhancement will
be achieved by, to lower the threshold, using a longer
POF at shorter wavelength such as 1.1 and 1.2 μm, whereits propagation loss becomes minimal (≅ 10 dB∕km). If a
1 km-long POF is used, its P th is calculated to be 472 mW,
which is comparable to the peak power under the measurement conditions above (Note that, to demonstrate thisexperimentally, most of the optical devices need to be
replaced with those suitable for the shorter wavelength).
When the duty ratio was reduced below 15%, theStokes power began to decrease slightly, as is recognizedin the inset of Fig. 6(b). The reason for this behavior,
which was not observed in our previous silica-based experiments [18], has not been clarified yet, but it might
originate from a longer rising time of acoustic phonons,
which is basically proportional to a phonon lifetime [21],in POFs. Although the Brillouin linewidth, which is in inverse proportion to the phonon lifetime [1], of POFs isreported to be ∼100 MHz [10,16] [also see Fig. 6(a)]and is broader than that of silica SMFs (typically, several
tens of MHz [1]), considering the multimode nature of
the POFs, we cannot simply conclude that the phonon
lifetime in POFs is shorter than that in silica SMFs. This
point needs to be studied further.
In conclusion, first, we proposed an MMF-assisted
POF-to-SMF coupling technique, with which damage orburning at the conventional butt-coupled POF/SMF interface can be avoided. Then, by use of this coupling technique, we demonstrated at 1.55 μm that the Stokes power
in a POF can be enhanced by using pulsed pump and an
EDFA. In the experiment, 4 dB enhancement of the
Stokes power was obtained. This amount can, in principle, be increased by employing a kilometer-order-long
POF at shorter wavelength to lower the Brillouin threshold. The Stokes power dependence on duty ratio was
nonmonotonic, which might be caused by a longer phonon lifetime in POFs than that in silica SMFs. We believethat the MMF-assisted coupling will be a standard POFto-SMF coupling technique for high-power light injection(including, but not limited to, Brillouin applications)with its low cost and capability of minimizing the risk ofburning at the POF/SMF interface. We also hope that theBrillouin signal in POFs enhanced with the pulsed-pump

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Fig. 6. (a) Measured BGS dependence on duty ratio and(b) Stokes power as a function of duty ratio. The inset in (b)shows the measured BGSs with 5% and 15% duty ratio.

technique will in the near future be of great use incharacterizing their BGS in detail, and that our result will
trigger further research on Brillouin dynamics in POFs.
This work was partially supported by a Grant-in-Aid for
Research Activity Start-up (24860029) from the JSPS, andby research grants from the Hattori-Hokokai Foundation,
the Mazda Foundation, and the JFE 21st Century Foundation.
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